Steam Turbines

The basic principle of the turbine is as old as
the wheel, linear energy is directed onto deflectors mounted on a central
shaft and rotational energy is imposed on the shaft. It is the basic principle
of the windmill and the waterwheel.

By the late 1800's the steam reciprocating engine
had reached it's pinnacle, there seemed to be no further avenue to explore
to improve efficiency, to get more power the engine had to be bigger,
and the size of the moving componants was imparting vibration both to
the engine, causing it to breakdown, and to the ship, which was particularly
unwelcome in lavishly decorated ocean liners.

Turbines of one description or another had been
experimented with, Watt himself had considered it and discounted it. The
problem was one of basic engineering: the reciprocating engine utilises
the pressure of steam, but the turbine principle uses the speed of steam,
and that is fast, 2,000 mph is fairly typical of a moderate power boiler.
In order to utilise that energy the turbine blades have to rotate at least
have the speed of the steam jet. Even by the 1880's it was just not possible
to construct a device that could rotate at those speeds without melting
or flying apart, probably both.

The turbine transforms linear energy, such as wind, into rotary energy.

Charles Parsons patented the first
workable turbine in 1884, and his genius looks simple in retrospect, like
all great ideas. He decided since he could not create a device that could
rotate at the speed of steam, he would slow the steam down.

The speed of the steam jet is dependant on the
rate of expansion, it had long been known and built into reciprocating
engines, that a vacuum, or partial vacuum, at the exhaust end of the engine
would create a higher pressure ratio between input and output and so impart
more energy to the pistons, this vacuum is the main function of the condenser
in which cooling steam is used to create a drop in pressure below that
of atmosphere.

Parsons idea was to reverse that function and create
an exhaust that was pressurised, but still below that of the feed. This
effectively resulted in a smaller pressure drop between feed and exhaust
and a slower steam jet. By repeating this process a number of times most
of the energy from the steam jet can be extracted without the turbine
having to destroy itself.

A simple turbine schematic of the Parsons type, rotating and fixed stators
alternate and steam pressure drops by a fraction of the total across each
pair, the stators grow larger as pressure drops.

The
method he used still forms the basis of turbines today, including the
gas turbines used in jets and modern warships. Parsons took a tube and
down it’s inner length he set rings of angled blades. He then set
a cylinder in the tube which also had rings of angled blades. Steam is
fed into the tube at one end, passes through the first set of fixed blades
and hits the first set of blades of the cylinder at an angle. Rotational
velocity is imparted on the cylinder and it begins to spin. The steam
passing through the rotating blades hit the next ring of fixed blades
and it is this effect that impedes the steam, causing a pressure to build.
By careful design of the interacting blades the ideal pressure differential
is created to spin the cylinder. The steam passing on down the tube encounter
the next set of blades attached to the rotating cylinder and the process
repeats, in the diagram the blade sets get bigger each time, the principle
is the same as in the triple expansion engine, as the available steam
pressure drops larger areas are needed to extract the energy at best efficiency.

There is a very basic flaw in the turbine that
may be obvious at this stage: since the interaction of the fixed and rotating
blades is critical to the efficient operation of the turbine it is not
possible to create a device with variable speeds. Later developments will
include a degree of variable pitch on the blades, but at this stage just
getting the blades to hang on is a challenge enough! Another problem is
the turbine will only operate efficiently at high speed, in the order
of thousands of revs, in order to fully utilise his turbine Parsons also
had to invent a better method of gearing down high speed shafts to a more
useful speed.

Ten years after filing his patent and with turbines
making their mark on land Parsons built the Turbinia, a 44 ton yatch a
100 feet long, 9 ft beam and 3 ft draught and demonstrated it at the 1897
Diamond Jubilee of Queen Victoria where it tore up and down the ranks
of warships at 34 knots when the fastest ships of the day were limited
to 27 knots.

Turbinia, preserved at the Discovery Museum in Newcastle,
to overcome cavitation she had nine propellers on three shafts.

The admiralty refused to invest in the
project even so but allowed Parsons to equip two new destroyers with turbine
engines at his own expense, they were HMS Viper and HMS Cobra which stunned
a sceptical audience by achieving 37 knots over a speed trial, Parsons had
only promised 30 knots.

Both Viper and Cobra
were lost in accidents at sea, confirming the doubters suspicions, but
Parsons again invested his own money and built another destroyer, HMS
Velox, and finally the admiralty agreed to equip a cruiser in 1902, HMS
Amethyst. Amethyst was one of four sister cruisers being built, the other
three received standard reciprocating engines and the performance comparison
deeply shocked even the conservative admiralty, a committee was formed
(of course) to look into the matter and in 1905 recommended that all future
warships be equipped with Turbines. Jackie Fisher had become First Sea
Lord the year before and heartily agreed, rushing through his concept
of an all big gun battleship and marrying it with turbines to create HMS
Dreadnought in 1906.

The value of the turbine cannot be measured in
speed alone, power for power the turbine was lighter and more compact
than the reciprocating engine and with less vibration made for a stabler
gun platform which effectively increased the accuracy and range of the
guns. At top speed the reciprocating engine was at the limit of it's capability
and suseptable to mechanical failure, but the turbine reached it's peak
effieciency at top speed.

HMS Amethyst, the first cruiser to be fitted with turbines.
HMS Dreadnought, the ship that re-wrote the concept of a Battleship

This activity was not lost on the
great merchant entrepreneurs of the time. In 1901 the King Edward became
the first Turbine Passenger vessel, operating on the Clyde, several smaller
ships followed but it was in the big liners that the Turbine proved it’s
full worth. These ships operated at full power for days on end, their
coal consumption per ship was equivalent to that of the entire Royal Navy
Home Fleet! The Virginian and Victorian were the first turbine equipped
liners, each of 13,000 tons, they were followed by the 30,000 ton Cunard
Carmania, she had a sister ship the Caronia and the comparison in speed,
fuel consumption and engine space occupied spelled the death of the reciprocating
engine in high performance ships. Lusitania and Mauretania of 38,000 tons
were to follow with Turbine engines giving 70,000 SHP.

Although the turbine was not efficient in slower
cargo ships and never fully replaced the reciprocating engine there it
did augment many. Parsons invented a turbine which utilised the waste
low pressure steam of these engines, geared down to existing shafts the
turbine improved fuel economy on long journeys in the order of hundreds
of tons of coal per voyage. This "Parasitic" Turbine was also
sometimes used to drive a seperate shaft, as on the Titanic whose central
shaft was turbine driven and the outer two reciprocating engines.

Crankshafts for the Titanic's reciprocating engines,
the sheer weight of the moving parts in the high power marine reciprocating
engines caused problems with vibration, wear and reliability, comparable
turbines were a fraction of the size.

A significant problem with the turbine
as a maritime engine was discovered quickly by Parsons when his Turbinia
failed to achieve the expected speeds in her first trials. Convinced his
engine was right he turned to the propellers and constructed a glass tank
and a strobe light to study the effects of the propeller at high speed.
He soon discovered what is now known as Cavitation, at the 2,000 RPM output
by Turbinia the outer tips of the propellors were turning so fast they
were unable to form a grip on the water and were generating pockets of
vacuum instead, in extreme circumstances the whole propeller would simply
spin around and produce nothing but bubbles, no movement at all.

Modifying the propeller design helped, as did using
smaller propellers, in the end Parsons fitted nine propellers on three
shafts in order to goose the Turbinia along at 34 knots. But in practical
use the only option was to run the turbine at less efficient slower speeds
and use small propellers. In warship design this was acceptable as the
advantages were great still, not least the ability to fully mount the
engines below the waterline, getting weight down low in the ship and providing
better protection. But as an example a coal burning triple expansion engine
would use 1.54 tons of coal per hour for every SHP produced, at low speed
a coal fired turbine was up to 2.4 tons as opposed to 1.2 tons at high
speed.

The turbine needed to be geared down, but gear
wheel manufacture at the time was not up to the job and Parsons himself
had to invent a new method of manufacturing gear wheels before they were
of sufficient precision to handle the speeds involved.

Part gearing (no, I don't know what that means
either!) was fitted to the Acheron Class Destroyers HMS Badger and HMS
Beaver in 1911 and then full single reduction gearing to the Laforey Class
Destroyers HMS Leonidas and HMS Lucifer in 1913, when employed in the
latest Dreadnoughts speed was increased to such an extent, despite heavier
armour and guns, that a new class came into being, the Super Dreadnought.

Typical arrangement of a High Pressure and Low Pressure Turbine through
a double reduction gearbox. A seperate turbine was needed for reversing
the ship, this was almost always on the LP turbine where fitted.

The combination of gearbox and turbine
made the engine expensive, and they were inefficient at low speeds, though
the invention of the cruise turbine which was optimised to run at lower
steam pressures helped conserve fuel when ships were running at low speeds
helped. But in the merchant service only the big liners with their long
distant sprints across the oceans really utilised the turbine. The development
of the oil fired furnace was given a great boost during WW1 and between
the wars the oil furnace - reciprocating engine pairing challanged the
turbine, particularly in the area of fuel consumption.

To overcome this Parsons demonstrated the advantage
of using high pressure boiles, typical boilers of the time were at 200-275
psi, by doubling the pressure Parsons showed that turbine efficiency was
hugely improved. The RN developed the three drum 500 psi boiler which
became virtualy the standard fit for all warships up to and including
WWII, two such boilers married to two sets of Parsons turbines with a
Low Pressure and High Pressure turbine in each set would typically generate
40,000 SHP and push an unladen light Destroyer of the era along at 37
knots with dual reduction gearing. By comparison HMS Hood had 24 boilers
and four turbines on four shafts generating just over 150,000 SHP which
in 1920 shoved her 45,000 tons along at 31 knots.

An alternative to mechanical gearing was Electric,
essentially the turbine was coupled to a generator which in turn powered
an electric motor, giving much better control, but at a cost in efficiency
due to the inherent losses in both generator and motor. But the system
had it's uses and the Buckley Class Destroyers of the USN used a similar
system in WWII, some of which were lend leased to Britain as Captain Class
Frigates.

The actual shape and layout of a turbine varies a great deal, but this glimpse
under the hood of a a high pressure turbine gives a rare peek.

Before I move on I need to cover
something of the different types of turbine.

Impulse or De Laval

In this type of turbine the operation is rather
more like that of waterwheel. In the waterwheel water is directed into
buckets which fill, impart rotational velocity by gravity and then empty
to rise and collect more. The impulse turbine has ring of fixed nozzles
that blast onto bucket type vanes of a rotating wheel. The pressure drop
is achieved in the nozzle which is flared and not by the rotating stator.
the de Laval spings at typically 30,000 rpm and is ineffiecient at using
the steam energy.

Reaction Turbine

The interaction of alternate fixed and rotating
stator wheels forms nozzles as the blades align with each other, the moving
stator is primarilly moved by the steam expanding into the virtual nozzle
and then being forced by the shape of the blade to change direction, hence
"reaction."

Impulse-Reaction or Compound or Parsons

In this type of turbine the blades are shaped to
form a cross section inducive to impulse drive at the base and reaction
at the tip. The Parson's turbine has a distinctive shape as the high pressure
stators are smaller than the low pressure stators resulting in fan effect,
in larger turbines the stators are mirrored and the steam fed to the centre
to split either side and so reduce stress on the system.

Velocity Compound or Curtis

Curtis combined the de Laval and Parsons turbine
by using a set of fixed nozzles on the first stage or stator and then
a rank of fixed and rotating stators as in the Parsons, typically up to
ten compared with up to a hundred stages, however the efficiency of the
turbine was less than that of the Parsons but it operated at much lower
speeds and at lower pressures which made it ideal for ships, the Curtis
turbine was manufactured under license in Britain at John Brown's shipyard
on Clydebank, hence the Brown-Curtis Turbine. Note that the use of a small
section of a Curtis, typicaly one nozzle section and two rotors is termed
a "Curtis Wheel"

Pressure Compund Multistage Impulse or Rateau

The Rateau employs simple Impulse rotors seperated
by a nozzle diaphragm. The diaphragm is essentialy a partition wall in
the turbine with a series of tunnels cut into it, funnel shaped with the
broad end facing the previous stage and the narrow the next they are also
angled to direct the steam jets onto the impulse rotor.

Now just to complicate things a typical maritime
turbine set would be any mix and union of the above types. A marine turbine
often traded off maximum efficiency to conserve space and weight, in particular
the large low pressure turbine wheels of the Parsons type could be dispensed
with for only a few percent loss of power and a gain of considerable reduction
in weight and size of the turbine.

Principle of the basic Impulse Turbine
Curtis Wheel, typical arrangement is two rotating stators with one or
more sets of blades, a stationary stator and a bank of fixed steam nozzles,
used to extract power from the initial stage in a high pressure engine.
Rateau stage, each stage has a set of nozzles and an Impulse Wheel

A typical compact geared marine turbine. The gearbox
is a signficant proportion of the engine and in some cases the largest single
unit. The condenser is cooled with sea water in a process that is the reverse
of what happens in a boiler. Sea Water is highly corrosive and "Condenseritus"
is the old demon of steam ships where sea water gets into the feed water
chain via corroded pipes and contaminates the engine and boiler.

Dual Axial Low Pressure Turbine with Reversing Turbines. Steam is fed to
the centre of the Parsons Compound turbine and released through pairs of
static and rotating blade wheels before venting to a condenser. To go in
reverse the steam is diverted to a pair of reversing turbines, often a Curtis
Wheel as it provides the most power for a compact unit.